3D mapping of a scene by structured light triangulation comprises illuminating the scene with a light pattern and observing the illuminated scene with a camera whose optical axis is offset from the illumination source. If a given ray from the illumination intersects a reflective object, an image of this ray will be formed on the camera. The location of the image of this ray together with knowledge about the exact geometry of illumination-imaging setup allows the determination of the relative position of the intersection between the light ray and the object. This supposes that one knows which ray of the structured light pattern intersected the object. Finding the ray of the pattern, which corresponds to a point in the image, is often called the correspondence problem.
In order to solve this problem, it has been proposed to generate a spatially coded structured light by an array of laser diodes. The laser diodes may be of the surface-emitting type, where the light is emitted in the direction perpendicular to the semiconductor wafer surface. Plural such laser diodes are integrated monolithically to form an array. The position of the individual laser diodes in the array is coded spatially to form a non-regular unique pattern.
The challenge with this type of illumination unit is to distribute the individual light sources in such a way on a given chip area, that the pattern of the positions of the light sources in any given subarea of the chip area is unique.
Apart from spatial coding of the structured light field, structured light imaging applications require typically a specific field of view (FOV) for the system, a certain range of distances, over which the system should operate accurately, as well as a specific minimum size of details in the scene objects which should still be resolved. These requirements vary from one application to the other.
For some applications, for example, it would be useful, if the distance range, over which the system should operate, could be set by design to be varying with the angles spanning the FOV. Similarly, in some applications an optimized structured light field would allow for a varying resolution over the field of view, that is to say a varying graininess or a varying density of features in the structured light field.
For structured light generators based on arrays of light sources a spatial coding as well as special requirements as just mentioned could in principle be fulfilled by using a dedicated arrangement of the light sources in the array and projecting the output of this array into the FOV using an appropriate projection optics system. However, for cost reasons, and because of diverse technical limitations the size of such an array of light sources and the freedom to arrange the light sources within the array is limited, such that the flexibility to design a system to the requirements of the application in this way is too limited.